Of significantly more concern is the possibility that the eyepiece itself employs a thoriated lens. This has been the case in some military devices and some unauthorized commercial devices. This is a much greater problem because of the proximity of the eyepiece to the eye and the fact that alpha and beta emissions now contribute to the dose. If the eyepiece is radioactive, the germinal cells of the cornea become the tissue of concern and the potential annual dose becomes quite high. NUREG 1717 discusses this in some detail. Fortunately, thoriated eyepieces are comparatively rare.
Eyepiece lenses coated with thorium, a radioactive material, can cause cataracts.